struct ListNode
{
    int val;
    ListNode *next;ListNode() : val(0), next(nullptr) {}
    ListNode(int x) : val(x), next(nullptr) {}
    ListNode(int x, ListNode *next) : val(x), next(next) {}
};

class Solution
{
public:
    ListNode* swapPairs(ListNode* head)
    {
        if(head == nullptr || head->next == nullptr)
            return head;
        ListNode* newhead = swapPairs(head->next->next);
        ListNode* ret = head->next;
        head->next->next = head;
        head->next = newhead;
        return ret;
    }
};